In the printing industry it is necessary to be able to obtain a proof. A proof is an accurate representation of the appearance of a final printed image. The proof can be used to check that the results of the print run will be exactly as desired before the print run is set up. As setting up a print run can be very expensive, the availability of accurate proofs can save much time and expense. The need for very accurate proofs is particularly strong in the area of color printing.
The image produced by a printing press generally consists of halftone areas which have a very large number of small dots of ink, printed onto paper. The dots in the halftone areas are called "screen dots". Screen dots typically have sizes in the range of 0.1 to 0.2 mm. In addition to halftone areas made up of screen dots, the printed image may also have solid color areas.
The appearance of a halftone printed image is determined by the size and positions of the screen dots, the intrinsic characteristics of the ink used and the thickness of the ink layer deposited in the printing process. It is this appearance that a proof attempts to predict. In producing a proof, the goal is to match the apparent color of each area in a proof as closely as possible to the apparent color of the corresponding area of a final printed image.
Devices called "halftone color proofers" or "dot proofers" are currently used in the printing industry to provide color composite images which predict the end result of a printing process. Most available halftone color proofers operate by transferring a colorant from a sheet coated with a uniform layer of colorant (which may be called a "donor") to a substrate such as paper, plasticised paper, or another suitable substrate. In this specification the term "colorant" includes pigments, dyes and other coloring agents which can be transferred to a substrate to produce a proof. Modern halftone color proofers typically use a laser to transfer the colorant from the donor to the substrate. This is done by placing the donor sheet close to a substrate and focussing laser energy on a small area of the donor sheet. The energy causes colorant in the small area to be ejected from the donor sheet and to be deposited onto adjacent parts of the substrate. The laser power is turned on and off as the laser is scanned across the donor sheet. The colorant forms a patterned layer on the substrate.
The apparent color of a portion of a proof is determined by the area coverage of screen dots in that portion, the characteristics of the transferred colorant and the amount of colorant transferred (e.g. the thickness of the layer of transferred colorant). In general the optical density of the colorant used in a halftone color proofer will be different from the optical density of the ink used to print the final printed image.
It is desirable to operate a halftone color proofer in "saturation mode" or "binary mode" wherein, in each pixel on the proof, either a maximum amount of the available colorant is transferred from the donor onto the substrate or no colorant is transferred. An advantage of operating in binary mode is that it is not necessary to maintain precise control over laser output. Small fluctuations in laser output power will have little effect as long as the laser retains the ability to transfer a maximum amount of colorant to a pixel on the proof.
In prior art binary mode color proofers, the apparent color of an area on a proof is adjusted by changing the sizes of the screen dots in that area. A significant disadvantage of operating in binary mode is that changing the sizes of the screen dots to sizes which are different from that of the final printed image can subtly, but significantly, alter the appearance of the proof. Prior art halftone color proofers have particular difficulty in matching apparent colors in solid color areas, where there are no screen dots.
Another type of laser halftone proofer varies the thickness of the layer of colorant which is transferred to the substrate in an attempt to match the optical density of the colorant to the optical density of the ink to be used in the printing press. Such systems typically use variable power lasers to vary the amount of colorant transferred to each portion of the substrate. Such systems allow accurate matching of ink optical density but are prone to calibration errors. Since the thickness (and consequently the optical density) of the transferred colorant varies with laser power output, any fluctuation in the laser power output will result in undesired fluctuations in the optical density of the deposited colorant. Another disadvantage of proofers which use a variable power laser is that laser beams typically have a Gaussian power distribution profile. Increasing laser power tends to increase the area of a donor sheet affected by the laser beam which, in turn, tends to increase the size of printed features.